Ghostscript executable not found. You will not be able to generate PDF using PDF (Ghostscript, level 1.2) or PDF (Ghostscript, level 1.4) options, as these output methods require GNU Ghostscript or AFPL Ghoscript to be installed on your server.

Ghostscript not installed on your server Install Ghostscript 7.xx or 8.xx yourself or ask your system administrator. Please note that if you're using some kind of package manager, you'll need both 'ghostscript' and 'ghostscript-fonts' packages for PDF generation (some ghostscript distributions do include fonts; some don't).
Installed Ghostscript executable have a non-standard name Modify GS_PATH configuration variable to match the real name of Ghostscript executable. Say, if you're using Windows-based server, you probably have Ghostscript executable named gswin32c.exe instead of more common gs; in this case, set the value of GS_PATH to 'gswin32c.exe' (NOT gswin32.exe!)
System search path does not include path to Ghostscript executable. Either modify the system search path, or provide full path to Ghostscript executable; for example, set the GS_PATH value to '/usr/local/bin/gs' (or whatever the real path to Ghostscript on your server is). Please note the difference between your user profile and user profile PHP is running under! While you may have Ghostscript on your default search path, nobody or apache (or whatever the name of system account running web server is) may have different search path not inluding the ghostscript directory.
You're using PHP in safe mode and Ghostscript is not on your safe_mode_exec_dir (please, refer to your PHP documentation or for explanation of these terms). Turn PHP safe mode off yourself or ask your system administrator.
Add path to Ghostscript executable to your safe_mode_exec_dir PHP configuration variable (or ask your system administrator to do it)